A new electromagnetic (EM) calculation model, the inhomogeneity zonal medium model (based on the physical optics (PO) method), is proposed to calculate and analyze the EM scattering characteristics of hypersonic target aircraft covered with a plasma sheath. First, the flow field of the plasma sheath is divided into non-uniform calculation subregions with different spacing sizes. The shape of the external flow field of the aircraft is modeled by fitting with a paraboloid and circular platform with different radii. Then, the electron density, temperature, and pressure distribution are obtained by collecting data from different points in the flow field of the sheath. Finally, a piecewise PO method is used to calculate the backscattering radar cross section (BRCS) of the hypersonic vehicle covered with plasma sheath from L-band to X-band. The numerical results show that the EM scattering characteristics of a target aircraft covered with a plasma sheath can be described more accurately by using zoned medium model simulation. Under different flight conditions, the BRCS of hypersonic aircraft covered with a plasma sheath appear to be reduced to different degrees in the S-band and the band range of 7-11GHz, and a large attenuation of BRCS occurs in the head area of the aircraft. The research results enrich the EM modeling method of hypersonic vehicles, overcome some shortcomings of the existing PO method, and provide technical support and innovative ideas for the accurate analysis of the EM scattering characteristics of hypersonic targets.

Based on the detection platform of space-borne radar (SBR), the dynamic electromagnetic (EM) scattering characteristics of a hypersonic vehicle covered with plasma sheath are calculated and analyzed by the physical optics (PO) method. Using the angle-time conversion method, the dynamic radar cross section (DRCS) of the vehicle in horizontal flying path and different flying states is solved in the range of L-band to X-band. The relevant effective statistical features are extracted from the time-scale matrix of DRCS based on continuous wavelet transform (CWT). The numerical results show that: At different flying altitudes, the backscattering radar cross section (BRCS) of the hypersonic vehicle covered with plasma sheath has irregular fluctuation and large range reduction in the head region of the vehicle. Generally, the higher the incident EM wave frequency is, the more detailed the fluctuation characteristics of the DRCS can be reflected. Within the same radar illuminate angle range, the time consumed by the vehicle DRCS is inversely proportional to the flying height and flying speed and is directly proportional to the detection altitude of the SBR. Besides, the different kinds of statistical eigenvalues extracted from DRCS will play an important role in hypersonic targets classification and recognition.

Using the parallel physical optics method, the range profile of a hypersonic vehicle with an electrically large size covered by 1-D inhomogeneous plasma sheath was studied. In this article, under the condition of far-field approximation, the improved physical optical method of 1-D nonuniform plasma coating of the conductive target was applied to calculate the scattered electric field of the target at different frequencies; then, 1-D Fourier inversion of the scattered electric field was carried out to obtain the 1-D range profile of the target. Numerical results showed that the temperature and pressure changes inside the plasma will have an impact on the range profile of the aircraft, and the intensity of the range profile of the target is inversely proportional to the temperature and pressure. In addition, the flight Mach parameters of the target aircraft, which affect the 1-D range profile, were also analyzed. The results showed that the larger the flight Mach number, the easier it is to detect the target aircraft. Finally, it was found that the nonuniformity of the plasma significantly influenced the 1-D range profile of the coated target; the intensity value of the range profile of the target aircraft (covered by the plasma sheath) at different flight heights was significantly lower than that of the pure metal target, resulting in the so-called “stealth” effect. This research provides some methods for effectively measuring and controlling hypersonic vehicles in adjacent space and alleviating the interruption of reentry communication.

In this article, a hypersonic target electromagnetic (EM) scattering echo model combined with the inhomogeneous zonal medium model (IZMM) and the classical scattering center model (SCM) is proposed with a distributed satelliteborne array radar as the detection platform. A parallel physical optics (PO) method is used for multiview inverse synthetic aperture radar (ISAR) imaging of a moving hypersonic target covered with plasma sheath based on the analysis of high-resolution range profile in the S-X ultrawideband range, reconstructing 2-D EM scattering echo data (the target) and motion compensation. The results show that the surface of the inhomogeneous plasma sheath flow field is an excitation layer with random and irregular fluctuation characteristics, which increases the false scattering centroid of the 1-D range profile of the hypersonic target and can interfere with and disrupt the radar localization of the target along the radial direction. In addition, shallow scattering of EM waves occurs in the plasma sheath, and the average signal intensity of the target can gradually reduce from 0.5 x 10⁻⁵ at 60 km and 20 Ma to 0.1 x 10⁻⁵ at 30 km and 20 Ma, with a fivefold weakening of the overall scattered echo signal. In particular, the faster the hypersonic target travels at 30-km altitude, the weaker the imaged scattered echo signal becomes, with the average intensity of the imaged signal weakening by approximately threefold from 15 to 25 Ma. This study provides considerable technical support and data assurance to establish a synthetic aperture radar (SAR) automatic target recognition (ATR) database, and the findings of this study can be used as a reference for fine-structure feature analysis of hypersonic targets for feature extraction and the classification and identification of targets.

The physical optics (PO) method based on subregional triangular patch division was used to study the electromagnetic (EM) scattering characteristics of a blunt cone hypersonic vehicle under terahertz (THz) waves in different environments. First, the backscattering radar cross section (RCS) of a metal blunt cone under gigahertz and THz waves was calculated separately; the results show that the RCS under the THz wave has higher accuracy and can better reflect the structural characteristics of the target. Then, the RCS of a blunt cone covered with a plasma sheath at different reentry heights was calculated; the results show that the existence of the plasma sheath will reduce the target RCS, and the degree of reduction is related to the maximum electron density, the frequency of incident waves, and other factors, but THz waves can alleviate this effect. Finally, the RCS of a blunt cone covered with a plasma sheath and radar absorbing material (RAM) at different incident EM wave frequencies were also studied, and some interesting conclusions are preliminarily obtained. Our work can provide a meaningful reference for advancing research on the stealth and anti-stealth capabilities of hypersonic vehicles.

This study aims to apply generative adversarial networks (GANs) to the effective classification of high/low resolution (HR/LR) image pairs obtained via inverse synthetic aperture radar (ISAR) for hypersonic objects covered with a plasma sheath. We propose a classification training model based on a Wasserstein GAN with a gradient penalty (U-WGAN-GP) framework, wherein a U-Net with an excellent jump connection structure is used as a generator, and a VGG-Net with high robustness is used as a discriminator, to support the reliable classification of HR/LR ISAR image pairs for the enhancement of ISAR image resolution. The WGAN-GP provides a shortcut for the gradient propagation of network parameters during the training stage through the stable encoder and decoder structure of U-Net. It inherits the echo intensity variation and position distribution characteristics of the scattering points between hypersonic objects in LR images and effectively transplants them into the generated super-resolution ISAR images, and it establishes end-to-end mapping between the HR/LR ISAR images. Moreover, the VGG-Net ensures that the generated super-resolution images are stable, controllable, and undistorted. In addition, the WGAN-GP structure combines the content and adversarial losses, optimizes the generator loss function, and uses the GP method to provide a stable and continuous training process. Finally, a traditional VGG-16 network classifier is added at the end of the training model. The proposed model is applied to an HR/LR image pair dataset of hypersonic objects. Compared with existing methods, the proposed method improved the classification and recognition accuracy from 54.4% to 81.8%.